You could always just take all the components out just to be extra safe, and use something to put the water in? if you dont, and happen to spill the water on something, as long as its not on, and you let it COMPLETELY dry, you should be fine.
Yes to be idiot safe remove your mainboard and everything. Otherwise just disconnect the power to your mainboard and I guess you have the refill screw underneith your case. You should also have got a jumper to jumpstart the PSU without having it attached to the mainboard to help drain the system.
Also when you drain it helps to either cut a tube or use something that let´s air get into or out the case when draining.
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